Cryogenic neuristor employing inductance means to control superconductivity



Nov. 16, 1965 M. W. GREEN GRYOGENIC NEURISTOR EMALOYING INDUGTANCE MEANS TO CONTROL SUPERCONDUCTIVITY Filed Sept. 30, 1963 \o 4 CONSTANT I {8 I CURRENT SOURCE k\2 TRiGGER PULSE. SOURCE 50 54A CONSTANT 58 CURRENT soURcE 52 J TmeeER PULSE. 6 SOURCE 15' J 56 *vmeeuz PULSE soURCE CONSTANT. CURRENT soURCE 2 Sheets-Sheet 1 24a 2% 24C 24d. 24 e [2O CONSTANT 28 I I CURRENT $OURCE 2z 4 TRIGGER DUL5E. $0URCE TRK5GER 44 PULSE 5OURCE CONTANT 48 40 CURRENT 5OLARCE.

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JMJ flow United States Patent CRYOGENIC NEURISTOR EMPLOYING IN- DUCTANCE MEANS TO CONTROL SUPER- CONDUCTIVITY Milton W. Green, Menlo Park, Califi, assignor to Stanford Research Institute, Palo Alto, Calif, a corporation of California Filed Sept. 30, 1963, Ser. No. 312,541 9 Claims. (Cl. 30788.5)

In an article by Hewett D. Crane, entitled Neuristor A Novel Device and System Concept, which appears in the October 1962 issue of the Proceedings of the IRE, on page 204, there is described a device which effectively comprises a distributed line with active and passive processes so distributed that a signal propagates along the line without attenuation, much as a burning zone moves along a fuse or an ionic discharge along a nerve axion. In the article, it is shown that lines of this type can be interconnected in certain direct ways to achieve a complete logic capability comprising networks of such lines alone.

In the article, Crane lays down certain requisite properties for a neuristor line. First, the line must be longitudinally symmetrical. Second, the velocity of the signals propagating through that line is uniform and de pends only on the properties of the line itself. Third, each portion of a line becomes refractory for some period after the passage of the discharge. A primary effect of the refractory interval is that some minimum spacing must exist between pulses propagating in the same direction along the line. A fourth property is that, two discharges which are propagating towards each other along a line are annihilated on collision. At the instant of collision, the line must be refractory on both sides of the collision point, thus neither signal can continue to propagate. Finally, a neuristor line must me inherently free of reflections, not only in the case of collisions but also when a discontinuity appears in the line.

An object of the present invention is the provision of a unique construction for a neuristor device.

Another object of the present invention is the provision of a simplified neuristor construction.

Yet another object of the present invention is the provision of a novel and useful cryogenic neuristor structure, which permits general logic configurations to be realized.

These and other objects of this invention may be achieved in an arrangement wherein a path which may take the form of a thin superconducting wire is connected in parallel with another structure which provides a low resistivity path with finite inductance. A constant current source is connected across the wire. A triggering signal is applied to the superconducting wire or path by any suitable means for raising the temperature above the superconducting level in a localized portion thereof. The resistance of this wire increases and due to current flow therethrough, it heats up thereby transmitting heat to adjacent regions of the superconducting wire, thereby lowering the critical field for neighboring portions of the wire. The heated up or normalsuperconductor interface will propagate down the wire. The Wire shunting structure, after an interval determined by its inductive impedance, serves to divert the current which is flowing through the superconductive path and which, as a result of the increased resistance, is causing heating of the wire. Consequently, eventually the wire temperature will drop and it Will recover superconductivity. Accordingly, such a structure meets the basic requirements for being fabricated into a neuristor line and can be employed for logical structures, such as are shown and described in the mentioned article.

The novel features that are considered characteristic of this invention are set forth with particularly in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic diagram of a neuristor element in accordance with this invention;

FIGURE 2 is a schematic diagram illustrating a neuristor line in accordance with this invention;

FIGURE 3 is a schematic diagram of another neuristor line arrangement in accordance with this invention;

FIGURE 4 is a drawing of a neuristor in distributed form in accordance with this invention;

FIGURE 5 is another drawing of a distributed neuristor arrangement in accordance With this invention;

FIGURE 6 illustrates still another variation of the distributed form of a neuristor, in accordance with this invention;

FIGURE 7 is a circuit diagram illustrating the coupling of two neuristor lines in accordance with this invention;

FIGURE 8 is a drawing of a distributed arrangement of FIGURE 7.

FIGURE 9 is a circuit diagram of a neuristor circulator circuit in accordance with this invention; and

FIGURE 10 is a circuit diagram of a unilateral structure using neuristor elements in accordance with this invention.

Some metals such as tin and lead are known to have the property of superconductivity. It has been observed that the resistivity of these metals disappears when they are superconductive and reappear when they are not superconductive. The temperature below which the superconductive property is exhibited varies with the particular metal. This temperature is called the critical temperature. It has been further noted that by applying a sufiiciently strong external magnetic field to a metal in its superconductive state, the resistive property of the metal reappears. This external magnetic field, which has been designated as the critical field, varies with temperature and vanishes at the critical temperature.

The required critical field can also be created by a current circulating in the superconductor itself. Thus, a superconductor cannot carry a current larger than a certain value, the so-called critical current, without becoming a normal conductor. The value of this critical current depends not only on the value of the critical field, but also on the size and shape of the superconductor.

Reference is now made to FIGURE 1, which is a schematic diagram of a neuristor element in accordance with this invention. A constant current source 10 supplies a constant current to a thin superconducting wire 12. The wire is maintained just below the critical superconducting temperature by any suitable cryogenic means, not shown. The current applied to the wire is just below the critical value. Assume further that an inductance 14 is connected in parallel with the wire and that the resistance of the inductance 14 is preferably less than the resistance of the wire across which it is connected when that wire is in its normal or non-superconducting state. The superconducting wire may be kept in a cryostat with external connections to afford the other structures to be connected in the manner shown, to the superconducting wire.

Assume now, that an additional magnetic field is applied to a portion of the wire 12, which is sufficient to make the value of the field at this portion of the wire exceed the socalled critical field value. This is provided by a trigger pulse source 16, which applies a pulse of current to an inductance 18 placed in proximity with a portion of the wire 12, which it is desired to trigger. The portion of the wire which receives the additional field goes normal. This portion immediately begins to heat up, lowering the critical field for neighboring portions of the wire. The normal region thus begins to grow and with it the resistance of the wire begins to grow. The normal region at first remains normal, despite being kept in the cryostat, by reason of the PR loss occurring in the normal region.

However, after a time determined by the inductance of the shunt inductance 14, some of the current flowing through the now normal superconductor path will flow through the inductance. The resistance of the shun-t inductance is selected to be such that when the wire goes normal, the inductance will shunt enough current to permit the wire to cool down. Consequently, the wire or path must eventually recover superconductivity. Thus, after a period of time, which includes the time required for the current flowing through the inductance to decay and again flow through the adjacent superconducting path, the cryogenic neuristor element is restored to its initial condition, ready to accept another triggering pulse. This period is the refractory period of the line.

FIGURE 2 is a schematic diagram of a neuristor line in accordance with this invention. A constant current source 20 applies current to a desired length of a superconducting wire 22, which has succcessive inductances 24a through 24c, connected in parallel with successive adjacent portions thereof. An alternative construction is to use a single multicapped inductance which is connected, as shown, to the super con-ducting line. The current applied to the line is just below its critical value. A trigger pulse which causes a magnetic field to be applied in excess of the critical value is applied to the line using a trigger pulse source 26 to apply a current pulse to an inductance 28. Inductance 28 is coupled to the line. Upon the occurrence of such trigger pulse, the portion of the superconducting line which is shunted by the inductance 24a heats up and this causes a division of current through the inductance 24a. The current fiow through the line under inductance 24b, 0, d and e remains as before since the total current is the same, except that the division through the first section of the line which has been caused to go normal is different. The hot region which has been created as a result of the critical field being exceeded then propagates down the line, successively heating up the successive portions of the line under the successive inductances, while the preceding portions of the line return to their previous superconducting state. Thus the hot region propagates down the line at a velocity of propagation which is determinable. The portion of the line which is left behind the advancing hot region successively returns to the cryogenic state after an interval, whereby the line is restored.

It can be seen from the foregoing description that the cryogenic neuristor elements in accordance with this invention possess the following properties:

First, a minimum stimulis threshold (the critical value) must be exceeded in order to trigger the line.

Second, the propagation through the line is substantially unattenuated.

Third, there is a uniform velocity of propagation.

Fourth, following the passage of a discharge past any point in the line, there is a refractory period for a brief interval as determined by the value of the parallel inductance.

It should be noted that the line can be triggered either by exceeding the critical field at one point or by increasing its temperature in any other suitable manner, as by focusing a high energy beam on the wire, or by bringing a heated object into proximity thereto.

The velocity of propagation is the velocity of the interface between the normal and superconducting regions. This velocity is constant since the interface always has a current I in front of it. Since there is an interval following the triggering on a section of line during which it transfers to the normal region and then back to the superconducting region, there is a refractory period during which an additional trigger has no effect on the line section which has just been triggered.

FIGURE 3 is a circuit diagram illustrating another embodiment of the invention which essentially is identical with that shown in FIGURE 2. The constant current source 30 supplies current just below the critical value to a desired length of superconducting line 32. In place of the succession of inductances connected in parallel with successive portions of the line, there is provided a tapped line 34. The tapped line 34 is selected so that each tapped portion thereof has a resistance such that when the associated parallel section of superconducting line is rendered normal, it will shunt enough current to enable the line to return to its superconducting state. Each section of the line 34 has a value of inductance such that the current shunt occurring when the parallel connected line section goes normal does not occur immediately, but rather take place over an interval.

Thus, effectively, when a pulse of current from the trigger pulse source 36 causes the trigger pulse coil 38 to apply an additional field to the first section of the cryogenic line 32, that section of the line becomes normal. However, in view of the inductance of the first section 34a of the line 34, current is not immediately diverted to the line section 34a. The triggered line section propagates the heat trigger to the adjacent line section. Meanwhile current increases through the section 34a until current fiow through the first section of the line 32 drops below what it was before. The first section of the line 32 begins to cool back to the cryogenic temperatures. The heat trigger meanwhile propagates down the line in an attenuationless manner, leaving a refractory line section behind.

FIGURE 4 illustrates a distributed type of structure which can be made to operate in the manner of the structures described in FIGURES l, 2 and 3. The superconductor wire 40 is attached to a substrate 42. The substrate can be any metal or resistive material which has the resistive property that it shunts sufficient current from the superconductive material when such material is normal as to permit the superconductive material to return to its superconducting condition. The substrate should have some inductance to provide a refractory interval. The superconductive material can be tin, lead or indium. The substrate can be copper, or other suitable material. The constant current source 44 applies current to the wire 40, and the trigger pulse source 46 applies a current to the winding 48 to trigger the line 40.

FIGURE 5 illustrates another distributed form of the cryogenic neuristor line. The superconductor here takes the form of a thin film 50 on a copper substrate 52. The copper substrate is made larger to provide the required resistance and inductive properties as well as heat dissipative properties. A source 54 applies a constant current to the film 50 and a trigger pulse source 56 applies a current pulse to the winding 58, when it is desired to trigger the line 50.

FIGURE 6 shows another configuration for an embodiment of this invention. This superconductive material 60 is formed on the substrate 62 in a spiral form in order to obtain more superconductor inductance per unit length of substrate. The reverse arrangement may be employed, namely, to deposit a spiral of resistor material on a superconducting core in order to obtain greater inductance per unit length for the resistive material. The invention will operate properly where the superconductor inductance is greater than that of the resistive substrate as long as the resistive substrate resistance is proper. The function of the inductive impedance is to prevent instantaneous switching of the current with changes of the superconductor between normal and superconducting states. This can occur regardless of which has the greater inductance. The constant current source 64 functions as indicated. The trigger pulse source 66 and the winding 68 also function in the manner previously described.

FIGURES 7, 8, 9 and 10 show how the cryogenic neuristor, in accordance with this invention, may be arranged to form characteristic neuristor type structures of the type described in the previously mentioned article by Crane. FIGURE 7 is a circuit diagram showing how one neuristor line can disable another neuristor line without initating a pulse in that line. Constant currents I and I flow in the opposite directions, as shown by the arrows, along the respective neuristor lines 70A, 70B, 70C and 72A, 72B and 72C. It will be seen that a number of inductors respectively, 74, 76, 78, 8t and 82, are connected between the cryogenic neuristor line 7013 and the cryogenic neuristor line 72B. Trigger pulses traveling down lines 70A and 703, which simultaneously reach common sections 7M3 and 723, coupled by the inductors 74 through 82, cause opposing currents to flow through the coupling inductors. These opposing currents cancel one another in the commonly connected regions of lines 79A and 70B. In effect, most of the current 1 entering the line 70A will be diverted to pass out of the network on branch 72C, while current entering the network on line 72A will be mostly diverted to branch 70C. Thus the central commonly connected paths '7ttB and 723 will be carrying currents substantially less than the normal values, I and I respectively. As a result, 1 R heating in these portions of the circuit will be insufficient to support a continuous normal region and both normal regions will recover superconductivity without further propagation. As a result, the trigger pulses cancel one another. A trigger pulse traveling down either line alone continues along that line without causing a trigger pulse on the other line. The sole effect on the other line is to temporarily inactivate the common portion of the other line until the common line section refractory period is over. This arises by virtue of the fact that the trigger pulse in the common line section causes an opposing current to flow up one coupling inductance and back through another further down the common section, thus lowering the current in the nontriggered common section of the line below its critical value.

FIGURE 8 shows a distributed version of the structure of FIGURE 7. Two thin superconducting films of tin respectively, 84A, 84B, 84C and 86A, 86B and 86C are deposited on a copper substrate 88. The common section 90 between line sections 84B, 86B operate in the manner of the coupling inductances 74 through 82, to cause a triggering pulse on one line to temporarily inactivate the other line during transition through the common section, and when two oppositely traveling triggering pulses simultaneously reach the common line section, they cancel one another.

FIGURE 9 shows a closed ring-line structure which, once a trigger pulse has been transferred thereinto, continues circulating the trigger indefinitely or until the power applied to the line is terminated. A cryogenic neuristor line 92 extends from point A which is the trigger pulse initiating point into an incomplete circular configuration 94. There is a region of thermal contact at the point 96 which is positioned between the end of the circular line portion 94 and the location in which the line 92 is connected to that circular portion.

Point 96 is merely a small region in which the two lines are in close proximity. There is no electrical connection between lines at point 96 but sufiicient thermal coupling eXists at this point so that passage of a trigger pulse along either line traversing region 96 will initiate a trigger pulse on the other line by raising its temperature. Note that just before line 92 reaches the thermal contact point 96, there are provided common coupling inductances 98, 100, 102.

Assume now that a trigger pulse has been initiated at point A on line 92. This trigger will be transferred along the line up to the point 96. In passing through the location of common coupling, it renders the circular line portion thereat temporarily refractory. Because of the thermal contact between the two line sections, the terminating portions 104 of the circular configuration may be triggered; however, this will continue to the end of the line and will not proceed backwards along the preceding portion of the line which is in its refractory stage. However, the trigger pulse will continue around the circular portion 94 in the direction of the arrow until it reaches the thermal connection 66. At this time, the trigger pulse will be transferred to the beginning of the circular portion of the line 94 again. Just before reaching the thermal point 96, the trigger pulse passes through the common coupling section again. The effect this time is to render that portion of line section 92 refractory whereby no trigger pulse is initiated along line 92. The trigger pulse will continue around the circular portion of the line 94 in the manner described. It will continue to circulate until interrupted by dropping the line current sufficiently low or by applying a trigger pulse to the common line section to cancel the circulating trigger pulse.

In order to insure that a trigger pulse is propagated in only one direction, a structure such as shown in the circuit diagram of FIGURE 10 may be employed. Assume that a trigger pulse is initiated at point A in FIG- URE 10 on the cryogenic neuristor line 110. This trigger pulse will propagate down the line reaching a portion IltlA wherein a number of inductors 112, 114, I16, 118 are connected between the line portion 11GB and a second line portion A. The common inductors have the same function as was described in connection with FIGURE 7. They serve, when the propagated trigger pulse on line 110A reaches the common line section, to divert some of the current I into line 120 to oppose the current I from source 119 in the common section, whereby the line 120 is temporarily disabled from transmitting a trigger pulse. The trigger pulse propagating along line Iii continues along a path which is equal to the path between the common section 120A and a thermal connection 122 as measured along the line 120. The thermal connection 122 initiates a trigger pulse on the line 12% which is propagated down the line to point B.

Should a trigger pulse be initiated at point B on line 12!) simultaneously with a trigger pulse being initiated at point A on line 110, it will not propagate completely down the line 120 in view of the coupling afforded by the common inductors 112 through 118, which operate to drop the current I flowing through the line 124) sufiicently far below the critical value to terminate further propagation of the triggering pulse. The triggering pulse ap plied to point B of line 120 alone will propagate down that line to the thermal junction 122, where line 116 is triggered. Trigger pulses then travel down both lines to the common section where mutual annihilation of the trigger pulse occurs. Thus, it can be seen that the configuration shown in FIGURE 10 operates to permit propagation from B to A and to block propagation from A to B.

It should be noted that a thermal junction between two neuristor lines can be made without them touching each other. They need only be sufhicently close so that the increase in temperature in one line triggers the other line. Alternatively, however, a thermal junction may consist of a simple branching of one line into two or more other lines. In the latter case, the currents in the branches would be less than that in the principal line. Thus, one neuristor might thermally branch to several lines of lower critical current.

From the foregoing description, it will be seen that there has been provided a novel and useful neuristor structure which may be termed a cryogenic neuristor. It affords structure for performing all the logical operations of which the neuristor is capable of performing, in a new and unusual manner.

I claim:

1. A cryogenic neuristor comprising a length of material having the property of supercondition whereby when in its normal state above a critical temperature value the resistance of said material is high relative to its resistance when in its superconducting state below the critical value temperature, said material being capable of being driven from its superconducting to its normal state by the application thereto of a current or magnetic field which exceeds a critical value, means for placing said material in its superconducting state, means for applying a constant current at an amplitude below the critical value to said length of material, means for causing a portion of said material to become normal, and means connected in parallel with said length of superconductive material having a resistance such that when said superconductive material is normal said means connected in parallel shunts sufficient current from said superconductive material to enable said superconductive material to return to its superconducting state and having an inductance value relative to the inductance of said superconductive material to delay the transfer of current between it and said superconductive material at the time said superconductive material switches between its normal and superconductive states.

2. A cryogenic neuristor as recited in claim 1, wherein said means connected in parallel with said length of superconductive material comprises an inductance having equally spaced taps therealong, said equally spaced taps being connected at equally spaced points along said material having the property of superconduction.

3. A cryogenic neuristor as recited in claim 1 wherein there is included a triggering circuit comprising an inductance in inductive proximity to a portion of said length of superconductive material, and means for applying a current pulse to said inductance to raise the magnetic field of said portion of said length of superconductive material above the critical value whereby said length of superconductive material goes normal.

4. A cryogenic neuristor as recited in claim 1 wherein said length of material comprises a film of superconductive material, and a substrate upon which said film is deposited.

5. A cryogenic neuristor as recited in claim 1 wherein said length of material comprises a wire made of superconductive material.

6. A cryogenic neuristor line coupling arrangement comprising a first and a second line, each including a superconductive wire, each wire having a first portion, a common portion and a third portion, a first and a second source of constant current, means connecting said first source of constant current to apply current flowing in one direction below the critical value to said first line, means connecting said second source of constant current to apply current flowing in the opposite direction below the critical value to said second line, four multitapped inductances a different one of which extends along and is connected to the respective first and third portions of the first and second lines, each of said inductances having a resistive value between taps such that it will shunt sutlicient current from the portion of said superconductive wire between taps when it is made normal to enable said wire portion to return to its inductive state, and a plu rality of inductances coupled between the common portions of said first and second wires at equally spaced positions therealong, each said inductance having a resistance such that when the common wire portion between connections goes normal, sufiicient cun'ent is shunted therefrom to permit the return to superconduction.

7. A cryogenic resistor line coupling arrangement comprising two cryogenic films deposited and extending along a substrate in the form of four extensions from a common junction section, each said film extending separately along one arm, the common section, and then along the oppositely extending arm, each said film being spaced and parallel to one anotheron said common junction section of said substrate to provide a common inductive cou pling region, said substrate having a resistive value such that when a portion of said film goes normal, it will shunt sufiicient current therefrom to permit said portion of said film to return to its superconductive state, first means for applying a constant current below the critical value to said first film flowing in one direction, and second means for applying a constant current below the critical value to said second film flowing in the opposite direction.

8. A cryogenic neuristor circulator comprising a line made of superconductive material, said line extending from a first point substantially in the form of a loop then in heating proximity to a portion of said loop and thereafter, to a second point, first inductance means connected in shunt with the portion of said superconductive line forming said loop except for the portion of said loop extending for a predetermined length adjacent said line extending from said thermal junction to said second point, second inductance means coupling said predetermined lengths of said line within said loop and extending from said thermal junction, said first and second inductance means having a resistive value such that when any portion of said line goes normal, it shunts sutficient current to enable said line to return to its superconductive state, and means for applying a constant current below the critical value to said line.

9. A cryogenic neuristor device comprising a first and second line of superconductive material, said first line extending a predetermined distance to a thermal junction portion, from thence along a curved portion, and from said curved portion along a common portion, said sec 0nd line extending a predetermined distance to a common portion opposite the common portion of said first line, and from said common portion along a curved path to a thermal junction with said first line at said thermal junction point, first inductive means connected in shunt with all of said first line except for its common portion, second inductance means connected in shunt with all of said second line except for its common portion, common inductance means coupling said first and second line com mon portions, means for applying a first current below the critical value to said first line flowing in one direction, and means for applying a second current below the critical value to said second line flowing in the opposite direction, said first, second and common inductance means having a value of resistance per unit length for shunting suflicient current when an adjacent unit length of line goes normal to enable it to return to superconductivity.

References Cited by the Examiner and System Concept by Crane, October 1962, pages 20482G60.

ARTHUR GAUSS, Primary Examiner. 

1. A CRYOGENIC NEURISTOR COMPRISING A LENGTH OF MATERIAL HAVING THER PROPERTY OF SUPERCONDITION WHEREBY WHEN IN ITS NORMAL STATE ABOVE A CRITICAL TEMPERATURE VALUE THE RESISTANCE OF SAID MATERIAL IS HIGH RELATIVE TO ITS RESISTANCE WHEN IN ITS SUPERCONDUCTING STATE BELOW THE CRITICAL VALUE TEMPERATURE, SAID MATERIAL BEING CAPABLE OF BEING DRIVEN FROM ITS SUPERCONDUCTING TO ITS NORMAL STATE BY THE APPLICATION THERETO OF A CURRENT OR MAGNETIC FIELD WHICH EXCEEDS A CRITICAL VALUE, MEANS FOR PLACING SAID MATERIAL IN ITS SUPERCONDUCTING STATE, MEANS FOR APPLYING A CONSTANT CURRENT AT AN AMPLITUDE BELOW THE CRITICAL VALUE TO SAID LENGTH OF MATERIAL, MEANS FOR CAUSING A PORTION OF SAID MATERIAL TO BECOME NORMAL, AND MEANS CONNECTED IN PARALLEL WITH SAID LENGTH OF SUPERCONDUCTIVE MATERIAL HAVING A RESISTANCE SUCH THAT WHEN SAID SUPERCONDUCTIVE MATERIAL IS NORMAL SAID MEANS CONNECTED IN PARALLEL SHUNTS SUFFICIENT CURRENT FROM SAID SUPERCONDUCTIVE MATERIAL TO ENABLE SAID SUPERCONDUCTIVE MATERIAL TO RETURN TO ITS SUPERCONDUCTING STATE AND HAVING AN INDUCTANCE VALUE RELATIVE TO THE INDUCTANCE OF SAID SUPERCONDUCTIVE MATERIAL TO DELAY THE TRANSFER OF CURRENT BETWEEN IT AND SAID SUPERCONDUCTIVE MATERIAL AT THE TIME SAID SUPERCONDUCTIVE MATERIAL SWITCHES BETWEEN ITS NORMAL AND SUPERCONDUCTIVE STATES. 